The disclosed multitone photonic oscillator is a very useful device for generating a set of RF tones with low phase noise and controlled tone spacing. It is a very compact device that can be used in a variety of RF photonics and wireless applications requiring a number of simultaneous carriers. Previous means for generating such low phase noise multitone carriers with controllable frequency intervals typically required a set of low phase noise RF synthesizers that are both bulky and expensive.
The generation of low phase noise multitone RF carriers is essential in a variety of radar and communications applications. Low Probability of Intercept (LPI) radar and communication systems use a set of rapidly frequency shifted low phase noise RF carriers as transmit waveforms in order to spread the information spectrum and hence reduce the probability of intercept. The presently disclosed device would be quite useful in such an application.
The disclosed multitone photonic oscillator can also be used as a source of low phase noise carriers in commercial wireless or radio on fiber applications. In these applications, the use of a compact multi-tone carrier generator for the frequency multiplexed wireless transmission of information could significantly reduce the size and cost of the currently used bank of RF carrier synthesizers.
Most prior art optoelectronic oscillators only generate a single tone. A photonic oscillator is disclosed by U.S. Pat. No. 5,723,856 and by a related article by S. Yao and L. Maleki, IEEE J. Quantum Electronics, v. 32, n.7, pp. 1141-1149, 1996. In this patent and the related article, a photonic oscillator is disclosed (called an optoelectronic oscillator by the inventors) which includes a modulator, an optical feedback loop, a filter and a photodetector. The aforementioned paper reports the generation of multiple tones achieved by enlarging the bandwidth of the filter and injection locking the oscillator to a continuously applied external RF signal. The spacing of the tones in this case is equal to the frequency of the injected signal. This method causes all of the oscillator modes (one tone per mode) to oscillate in phase. As a result the output signal is a series of pulses.
The present invention includes a modulator, an optical feedback loop, and a photodetector similar to the prior art. However, the present invention is different in that electrical injection locking is not used. Rather, injection seeding is used (to help initiate the generation of multiple tones) and the tone spacing is determined by the delay time of the optoelectronic feedback loop.
There are significant differences between the present invention described herein and this prior art:                1. In the present invention, the tone spacing of the multi-tone oscillations is not determined by the injection seeding RF signal (which may be applied only temporarily), but rather by the delay time of the optoelectronic feedback loop. In this prior art, the tone spacing is fixed and locked by the permanently injected signal.        2. In the present invention an injection seeding RF signal is injected into the oscillator in order to help excite a multitude of oscillation modes that have been suppressed due to the phase dispersion in the feedback loop by fine tuning their electrical phase. Once multitone oscillations have been established, the injection seeding RF signal may be removed and the oscillations are maintained. In the prior art, the external RF signal is permanently injected to lock the frequency interval of the oscillation modes. The prior art does not consider removal of the injection locking signal.        3. In contrast to this prior art in which the multiple tones are phase locked due to the injection locking signal hence resulting in a pulsed waveform in the time domain, the multi-tones of the present invention need not be non coherent. Thus, the output waveform is continuous in time.        